1. Field of the Invention
The present invention is broadly concerned with improved, high speed web or sheet processing apparatus designed for extremely accurate registration and operation upon successive material segments fed to the apparatus. More particularly, the invention pertains to such apparatus, and corresponding methods, which are operable for initially gripping or holding a fed material segment, whereupon the gripped segment is essentially simultaneously shifted along orthogonal axes within the plane of the segment, and about a rotational axis transverse to the segment plane for accurate alignment purposes. The invention is particularly suited for high speed accurate die cutting operations.
2. Description of the Prior Art
Three-axis die cutting presses have been proposed in the past for processing of continuous webs. One such press is disclosed in U.S. Pat. No. 4,555,968. The press of this patent includes a shiftable die unit supported on a cushion of air, and the die unit is moved laterally of the direction of travel of the web as well as rotatably about an upright axis perpendicular to the web in order to bring the die unit into precise registration with the defined areas of the web to the die cut by the press. Automatic operation of the press described in the ""968 patent is provided by a control system having two groups of photo-optical sensors which are disposed to detect the presence of two T-shaped marks provided on opposite sides of the web adjacent each defined area to be cut. The control system is electrically coupled to a servomotor mechanism for adjustably positioning the die unit once advancement of the web is interrupted in a defined area on the web in a general proximity to work structure of the die unit.
As shown in U.S. Pat. No. 4,697,485, a die cutting press is provided with a registration system operable to provide precise alignment of a shiftable die cutting unit along two axes during the time that the web material is advanced along a third axis to the die unit, so that as soon as a defined area of the web reaches the die unit, the press can be immediately actuated to subject the material to the die cutting operation. Continuous monitoring of an elongated indicator strip provided on the material enables the die unit to be shifted as necessary during web travel to ensure lateral and angular registration prior to the time that web advancement is interrupted.
U.S. Pat. No. 5,212,647 describes a die cutting press provided with a registration system that quickly and accurately aligns defined areas of a web with a movable die unit without requiring the use of elaborate or continuous marks or more than two sensing devices for determining the location of the marks relative to the die unit. The registration system of the ""647 patent employs a pair of reference indicia fixed on a bolster of the press for indicating the position at which the indicia on the web of material appear when the defined areas of the web are in a desired predetermined relationship relative to the die unit supported on the bolster.
Application for U.S. Letters Patent Ser. No. 08/641,413 filed Apr. 30, 1996, now U.S. Pat. No. 5,644,979, describes an improved die cutting press wherein the entire die unit comprising a lower platen and a shiftable, upper die assembly is supported on a cushion of air. During operation when a defined area of the web is initially fed to the die cutting station, the target area is gripped via a vacuum hold-down and the entire die unit is simultaneous adjusted along three axes so as to achieve precise alignment between the target area on the web and the die cutting assembly.
Although the accuracy provided by such prior art die cutting registration systems is very good, such presses are relatively slow. For example, in the case of the press described in the ""413 patent application the necessity of moving the relatively heavy and bulky die assembly tends to slow the operation thereof. The earlier die presses are in general able to operate at speeds no faster than about 20 strokes per minute.
There is accordingly a need in the art for an improved web or sheet-fed processing apparatus, such as a die cutting press, which avoids the problems of prior units of this type and gives very high speed registration and operation.
The present invention overcomes the problems outlined above and provides an apparatus and method for the processing of successively fed segments (i.e., portions of a continuous web or discreet sheets) so that operations such as die cutting can be rapidly and accurately carried out. Broadly speaking, the apparatus of the invention includes an operating station, means for initially feeding a segment of material into the station, and positioning means for accurately positioning the segment in the station after such initial feeding and prior to processing in the station. The positioning means includes segment gripping or holding means for firmly holding the initially fed segment, means for determining the position of the held segment within the station as compared with a desired position thereof, and motive means coupled with the segment-holding means for moving the latter and the segment held thereby to locate the segment in the desired position. Generally speaking, the material segments carry at least one and preferably a pair of position-identifying indicia, and the positioning means includes a reference assembly providing reference data corresponding to the desired position for the segment indicia, together with means for comparing the location of the segment indicia with the reference data.
In another aspect of the invention, an apparatus and method for processing of individual segments of a continuous flexible web is provided wherein accurate adjustment of the position of successively fed web segments is provided by initially holding each successive segment and subjecting the held segment to adjusting motion while the segment remains a part of a continuous web. This adjusting motion is selected from the group consisting of motion along either or both of orthogonal axes in the plane of the segment and rotational motion of the segment about an axis transverse to segment plane, and combinations of the foregoing motions. It is to be understood that the invention provides such three-axis movement of individually held web segments while the respective segments remain a part of the continuous web.
In preferred forms, the web gripping or holding apparatus of the invention includes a relatively lightweight vacuum hold-down plate within the web or sheet processing station. In the case of a die cutting press, the vacuum hold-down plate is in the form of a centrally apertured body surrounding an essentially stationary floating die cutting anvil; the vacuum plate is shiftable as necessary in an axial direction (i.e., in the direction of web travel), a lateral direction (transverse to the axial direction), and/or rotationally about an upright rotational axis perpendicular to the axial and lateral directions and to a plane containing the segments. As used herein xe2x80x9cdie cuttingxe2x80x9d refers broadly to encompass various operations including but not limited to stamping, cutting, punching, piercing, blanking, and other similar operations.
The preferred motive means is coupled directly to the vacuum plate and includes a plurality of spaced apart motors such as bi-directional stepper motors, each of the later being translatable during movement of the vacuum hold-down plate. In order to achieve the most accurate and rapid plate movement, the motors are coupled via eccentrics to the plate so that operation of the motors will drive and move the plate as required. In the most preferred form, the motive means includes three such eccentrically coupled stepper motors, with the axes of the plate-connecting shafts lying in a single, common rectilinear line.
The preferred positioning apparatus also makes use of a pair of CCD (charge coupled device) cameras mounted within the processing station, together with a pair of split prisms and fixed reference indices carried by the die assembly. In operation, when a material segment is fed to the processing station, each camera receives a combined image made up of an image of the fixed indicia as well as one of the fiducials carried by the material segment. This image data is then used to calculate registration error and distance of travel information which is in turn employed in the operation of the respective stepper motors, so as to move the vacuum plate and the material segment held thereby for accurate positioning of the segments.
The apparatus of the invention is similar to that described in U.S. Pat. Nos. 4,555,968; 4,697,485; 5,212,647 and pending application Ser. No. 08/641,41-3, all of which are incorporated by reference herein.